Method of manufacturing castings of cast iron of refined graphite structure



United States Pate t METHOD OF MANUFACTURING CASTINGS OF CAST IRON OFREFINED GRAPHITE STRUC- TU Hiroshi Sawamura, Sakyo-ku, Kyoto, Japan NoDrawing. Application May 15, 1952, Serial No. 288,049

C aims p y appl ion apan M y 9, 1 1

2 Claims. c1. 15-433 This invention relates to a new process ofmanufacturing eastings of cast iron eharacterized by a highlyrefinedeutectic graphite structure, the process involving the use of moltenslag containing at least 1.0% of titanium in the form of titaniumdioxide whichis caused to eontact molten pig iron or a bath of cast ironunder suitable conditions It is an objt ct of this invention to providea process for producing castings of gray cast iron which possessenhanced mechanical and structural properties.

It is another object of this invention to provide means facilitating ahighly economical production method for gray cast iron with pronouncedresistance to growth or graphite precipitation at elevated temperature;

It is further an object of this invention to provide means ensuringproduction of gray east iron of highly refined homogeneous eutecticstructure presenting considerably reduced permeability and preventingleakage under severe conditions of use, in particular when articles madeof such cast iron are subjected to gas-, airand hydraulic-pressure.

Th P e e t inve o co t l tes he prqvi iqn. o means enabling the use ofslag-forming material or molten slag with approximately 1.0% of"titanium or more in h arm f. a ta ium c m o d or e i a a t ea i s mo n Pi o o e t i qn unde P s ib o dit sx f. w ere after he t ment asse ssampl -id. he esu a mo en me a ma a in s nd m u ds. meta mo d r t e l kThe present invention is further direeted to castings 0f a ron a in aghly refine eu e t graphite structure, in contra-distinction to thecommon eoarse graphite structure (exemplified in Graphite FlaheTypeChart No A in A F. S.A. S. M Graphite Ciassificatiori in Gray CastIron).

In accordance with the practice of the present invention, molten; castiron, after having been treated with molten slag containing TiOz understrong reducing conditions, was cast in moulds and investigati o n ofits structure and composition revealed'the following facts:

(1) Where the titanium absorbed bythe cast iron melt from the moltenslag exceeds approximately 0.2% of said cast iron melt, substantiallythe entire casting presents a eutectic graphite structure.

Where the amount of Ti absorbed is less than about 0.2%, the treatedcasting is either of entirely common flaky r ph t uc ur a 1 f P r flakyt rh r c u e an a t euteetis rap it structure- (2) The content of N2 inthe cast iron has no effect upon the production of a homogeneouseutectic graphite structure throughout the entire casting.

In a word, the production of cast iron. characterized by a homogeneousand substantially completely eutectic graphite structure according tothe practice of the present invention appears to reside in the fact thatthe content of Ti in the treated cast iron slightly exceeds about 0.2%.

Patented Aug, 21, 195.6

Assuming that the temperature of a bath of cast iron which has normalcomposition aside from Ti is 1450""C., and, at this temperature,comparing the chemical afilnity of N2 or C to the various componentscontained in the cast iron, the chemical afiinity to Ti is thestrongest, (Kelley, U. S. Bureau of Mines Bulletin 407), andthe chemicalafiinity of C to Ti is stronger than that of N2 to Ti.

Moreover, the concentration of C in the bath of cast iron isincomparably greater than that of N2.

Therefore, also taking into account the result of analysis of N2 in thecast iron, it can be presumed that, in the bath of cast iron almost allof the Ti exists as TiC.

From the ternary equilibrium diagram of Fe-C-Ti by Tofaute-Biittinghaus(Arch, Eisenhiitten w., 12,193 8,33,), it is seen that the ternaryeutectic composed of austenite, TiC, and FesC is solidified attemperatures slightly below 1145 C., corresponding to a composition ofC=3.9% and Ti=0. 8%.

Although there is a considerable amount of various impurities" containedin the normally-composed cast; iron q l e ing m re h abou .2 of e is cosidered to take a more complicated solidifying curve than that shown inthe said diagram, the major portion of the eutectic formed when the castiron of this kind is solidified ispresumed to be mixed with theindependent solid phase of TiC.

It is understood that the graphite is refined because this TiC isscattered as very fine granules in the eutectic mixture and prevents thedevelopment of crystals of Pest? or of crystals of graphite, the latterresulting from decomposition of FesC.

It is still unknown why TiC prevents the development of 'crystals ofFesC and of graphite.

However, the latter effect seems to have something in common withphenomenon that when Ti is added to steel forming TiC, theaustenite-crystal-grains are refined (See: Comstock-Clark: Metals ofAlloys, 8, 1937,42, etc.) or the growth of austenite-crystal-grainscontaining TiC is impeded. ('See: Comstock: T. A. S. M., 281940608,etc.)

Even if the graphitization of cast iron were primarily brought about bythe crystallization of graphite from the melt, the reason for theresulting refined graphite structure can, be .,a5ily understood in termsof theexplauation given hereinabove in connection with the efiect of theaddition of titanium to steel.

As for the temperature of the molten pig iron and the molten slag oncontact, it is noted that this need not be particularly high since apreferred temperature range of 1 400.C. to 147.0" C. has provensatisfaetory.

The higher temperature limit may be more eifective for obtaining aeutectic structure because it will increase the amount of Ti which iscarried from the molten slag to the bath, and, consequently, increasesthe formation of fine granules of TiC in the bath.

It is further noted that it makes little difference whetherthe moltenslag and the bath are brought into contact at the same temperature, orthey are brought into contaet at somewhat different temperatures.

Oficourse, the bath of east iron and the molten slag are atsubstantially the same temperature in a furnace.

Several illustrative examples of the practice of this invention aregiven as follows:

Braw 1 In ordertomakaa. steel ingot-mould', 50 kg. of molten pig iron,which was melted in a 15-ton cupola, was cast in a green mould of 25 mm.dia.

The composition of the sample is as follows: C 3.7%, Si 1.5%, M11 0.6%,S 0.06%, P 0.24%.

Result: the graphite structure of the sample was of the common verycoarse flaky variety and its tensile strength is 14.7 kg./sq.mm.

Example 2 Immediately after 50 kg. of the same molten metal as inExample 1 above was put into a basic electric furnace (capacity: 250kw.), suitable amounts of clay, silica sand, and lime were chargedthereupon, forming a slag of the following composition: SiOz 37.8%, CaO46.0%, A1203 9.8%, MgO 4.3%, FeO 2.1%.

Then, while maintaining the molten pig iron at approximately 1400" C.,the molten slag was kept in contact with the molten pig iron for about15 minutes, and the resultant molten product was cast in a green mouldas above.

The amount of slag was 10% of the molten pig iron.

Result: the graphite structure of the sample tested was almost as coarseand flaky as that of Example 1, and its tensile strength was 14.9kg./sq.mm.

Example 3 A procedure similar to Example 2 was followed, with thedifference that ilmenite was added for the purpose of making a moltenslag of the following composition: SiOz 29.4%, CaO 48.3%, A1203 10.2%,MgO 2.8%, TiOz 8.0%, FeO 1.3%.

Result: The sample tested possessed a highly refined eutectic graphitestructure, and its tensile strength was as high as 22.8 kg./sq.mm.

Thus, cast iron having a highly refined eutectic graphite structure maybe obtained when the molten pig iron is cast in the mould aftersubjecting it to contact with molten slag at about 1400 C., which slagpreferably contains an amount of TiOz corresponding to at least 1% oftitanium.

In order to obtain such a cast iron product of highly refined eutecticgraphite structure, it is, of course, necessary to vary the contact timein accordance with the basicity of slag and the content of TiOz as wellas the total amount of slag.

For example, when the molten slag of normal composition is processed ina 250 kw. electric furnace, the following proportions and contact timesare preferred:

Basicity of Contact Amount of molten slag (ratio to TiOz molten slagtune molten pig iron), percent (percent) (OaQ per- (mincent/S102perutes) cent) The tensile strength of the samples obtained from theabove three examples was determined as follows:

Therefore, the tensile strength of the sample which was obtained usingthe method of the present invention is much higher than that obtained bythe normal method.

Comparing the graphite structure of the cast iron of Example 1 with thatof the cast iron of Example 3, the latter is much finer than the formerand accordingly, it is clear that the resistance of the latter sample ofcast iron to growth or graphite precipitation at elevated temperaturesis much greater than that of the former.

Consequently, the castings obtained by the practice of the presentinvention are particularly suitable for a mould for steel ingot whichrequires rigidity, toughness, and heatresisting qualities.

Furthermore, the present invention is most useful in manufacturingcastings of gray cast iron intended for exposure to an oxidizingatmosphere in high temperature range.

Moreover, it is observed that a casting having a finegrained eutecticstructure, as provided by the practice of the invention, is of such adense and compact nature as to minimize gasor water-leakage incomparison with a normal casting subjected to similar conditions of highairor high water-pressure.

Accordingly, this invention can be most eifectively utilized in themanufacture of castings of gray cast iron which are required to be gasorwater-proof.

Since certain modifications may be made in the process of the presentinvention, it is intended that all matter contained in the foregoingspecification be interpreted merely as illustrative and not in alimiting sense.

Having thus described the invention what is claimed as new and desiredto be obtained by Letters Patent, is:

1. The method of producing articles of gray cast iron comprising thesteps of maintaining molten cast iron at a temperature ranging fromapproximately 1400 C. to 1470 C., and subjecting said molten cast ironwithin said temperature range to contact with molten slag containing atleast 1% of titanium in the form of titanium dioxide, to thereby obtainupon solidification of the resultant cast iron an article offine-grained eutectic graphite structure and resistance to graphiteprecipitation at relatively elevated temperatures.

2. The method of producing articles of gray cast iron comprising thesteps of maintaining molten pig iron at a temperature ranging fromapproximately 1400 C. to 1470 C., contacting said molten pig iron withinsaid temperature range with a molten basic slag, said slag containing atleast 1% by weight of titanium in the form of titanium oxide and havinga calcium oxide to silica ratio in excess of 1, thereafter pouring saidtreated pig iron into a mold, and allowing said treated pig iron tocool, whereby upon solidification gray cast iron is obtained whereinsubstantially all of the carbon is present as eutectic graphite.

References Cited in the file of this patent UNITED STATES PATENTS609,467 Rossi Aug. 23, 1898 830,536 Slocum Sept. 11, 1906 1,955,791Comstock Apr. 24, 1934 OTHER REFERENCES Stahl und Eisen: vol. 60; Jan.4, 1940, page 13.

2. THE METHOD OF PRODUCING ARTICLES OF GRAY CAST IRON COMPRISING THESTEPS OF MAINTAINING MOLTEN PIG IRON AT A TEMPERATURE RANGING FROMAPPROXIMATELY 1400* C, TO 1470* C., CONTACTING SAID MOLTEN PIG IRONWITHIN SAID TEMPERATURE RANGE WITH A MOLTEN BASIC SLAG, SAID SLAGCONTAINING AT LEAST 1% BY WEIGHT OF TITANIUM IN THE FORM OF TITANIUMOXIDE AND HAVING A CALCIUM OXIDE TO SILICA RATION EXCESS OF 1,THEREAFTER POURING SAID TREATED PIG IRON INTO A MOLD, AND ALLOWING SAIDTREATED PIG IRON IS OBCOOL, WHEREBY UPON SOLIDIFICATION GRAY CAST IRONIS OBTAINED WHEREIN SUBSTANTIALLY ALL OF THE CARBON IS PRESENT ASEUTECTIC GRAPHITE.